The field of the invention is numerical control systems for machine tools, and more specifically, industrial processors which are suitable both for controlling axis position and motion on a multi-axis machine tool and for controlling the discrete, digital devices which are associated with the machine tool.
Numerical control processors have in the past taken the form of a hardwired, special purpose computer. Such hardwired computer controllers are interfaced with the servo control units on the machine tool to control the motion of a cutting tool along one or more axes and are interfaced with electromechanical relay panels or stand-alone solid state control circuits to control the discrete, digital devices which perform such auxiliary functions as tool changing and selection, pallet changing and selection, and coolant control. Each make and model of machine tool requires its own unique set of discrete digital devices and hence a unique control circuit must be designed to operate them. The machine tool builder has in the past been responsible for designing the relay panel or solid state control circuit and has been responsible for interfacing it with the numerical control system. These tasks are time consuming and add considerably to the overall cost of the machine.
As disclosed in U.S. Pat. No. 3,810,104, which is entitled "Programmable Magnetics for a Numerical Control System," the burden of interfacing a numerical control system with a particular machine tool has been substantially lessened by employing a programmable controller with the numerical control processor. The programmable controller may be easily programmed by the machine tool builder to operate the discrete digital devices on each machine tool, thus eliminating the time consuming and expensive design of special purpose hardward for each make and model. A numerical control system which employs the teachings of this United States patent is manufactured and sold by the Allen-Bradley Company, assignee of the present invention, and is identified as the Model 4500.
In more recent years general purpose digital computers have been employed as numerical control processors. "Mini-computers" such as the Model 2100A manufactured by the Hewlett-Packard Company and the Model PDP-8 manufactured by the Digital Equipment Corporation are programmed to perform numerical control functions and are interfaced both to the servo control units and the discrete digital devices on a machine tool. The programs which operate the minicomputer to perform both axis control and machine dependent logic functions are stored in the memory of the computer in machine language. To alter the operation of such a system to meet the unique requirements of a particular machine tool, therefore, the machine tool builder must employ skilled programmers who are knowledgeable of the entire "software system." In addition to the difficulties and complexities of real-time computer programming which this arrangement presents, the control of discrete digital devices with a general purpose digital computer results in considerable waste of costly memory space and valuable processor time.